Clicker question: Review from last class

1) Determine the answer to the following calculation using the correct number of significant figures.

A) 0.40 F) 4 x 107

B) 0.396 G) 40,000,000

C) 0.4 H) 4.0 x 107

D) 0.39 I) 3.9 x 107

E) 0.396220952 J) 3.96 x 107

Welcome to our CHEM 4 lecture• Go to LearningCatalytics.com Session ID =• While we wait, please start on the review question below.

(13.001)(640)

(2.10 𝑥 10−4)5sf

• Need to keep only 2 sf in the answer. • Leftmost dropped digit is “6” so round

up to 40,000,000. • Then report your answer with 2 sf using

scientific notation.

= 39,622,095.24

3sf

2sf

= 4.0 x 107

• If you answered A)-E), you forgot to put the denominator in parentheses in your calculator.

Clicker question: Review from last classGo to LearningCatalytics.com Session ID =

2) The length of the black stripe is between 11 cm and 12 cm as seen in the photo below. Which measurement is reasonable to record in your lab note book?

A) about 10 cm

B) 11 cm

C) 12 cm

D) 11.1 cm

E) 11.7 cm

F) 11.05 cm

G) 11.10 cm

H) 11.15 cm

• We are sure about the 11 cm because the smallest marking on the ruler is the 1’s place.

• Then we get a guess digit. It should be 1/10th of the smallest marking.• Our guess digit should be in the 10ths place. • Although 11.7 cm also has its guess digit in the 10ths place, it is not a

reasonable guess.

Key to Success in CHEM 4

✓ Visit our CHEM 4 website regularly: tinyurl.com/SacStateChem4

✓ Attend every lecture having completed the assigned reading.

✓ Review our PowerPoint slides and/or lecture recordings after each class.

✓ Keep up with daily homework. However, all students will automatically receive full credit for all late homework this semester.

✓ Complete all of the practice exams.

✓ Start formal studying for exams 1 week early.

✓ Talk to your Commit to Study peer mentor about how you are doing in CHEM 4.

✓ Get help when needed:

✓ Put together a weekly study group.

✓ Jeff’s office hours: MWF 9 – 9:30 am and 11 – 11:30 am; and by appointment.

✓ PAL office hours: link is on our CHEM 4 website.3

4

What do you want to do for week #9?A) Heat capacity (3.12 cont.) and review sessionB) Heat capacity (3.12 cont.) and climate changeC) Climate change and review session

Class voted on Monday

113 votes28 votes94 votes

So we’ll keep the calendar the way it is for October.

• When I have some free time (hopefully this weekend), I will record my climate change lecture and post it for students who are interested in learning about this important issue that impacts all of us.

• Watching the climate change lecture will be optional and you will not be tested on any material covered in the lecture.

5

The American biochemist Jennifer A. Doudna (UC Berkeley) and French microbiologist Emmanuelle Charpentier (Max Planck Institute) won the 2020 Nobel Prize in chemistry for their work on "genetic scissors" CRISPR (clustered regularly interspaced short palindromic repeats) that can cut DNA at a precise location, allowing scientists to make specific changes to specific genes.

They are the first women to jointly win the Nobel Prize in Chemistry, and the sixth and seventh women to win the chemistry prize.

Chemistry in the News

Jennifer Doudna (left) and Emmanuelle Charpentier pictured together in 2016.

Francis Collins, director of the National Institutes of Health, says, "You cannot walk into a molecular biology laboratory today, working on virtually any organism, where CRISPR-Cas9 is not playing a role in the ability to understand how life works and how disease happens. It's just that powerful."

CHEM 4 lecture

Wednesday – October 7, 2020

Sec 2.4 cont., 5.11

Formula mass

Reading clicker question (Covers material from today’s assigned reading)Go to LearningCatalytics.com Session ID =

3) Which of the following statements is true?

A) When adding and subtracting, the answer has the same number of significant figures as the measurement with the fewest sig. figs.

B) H2O has a greater formula mass than H2O2.

C) A compound’s formula mass depends on how much of the compound you have.

D) Formula mass is found by adding the atomic masses of all the atoms in a chemical formula.

E) Formula mass can only be calculated for substances with 2 elements.

7

Background: Determining answers for calculations (+/-)

8

For +/-, answers should have the same # of decimal places as the measurement with the fewest decimal places.

Example A: Example B: Add 1.009 m + 1.2 x 103 m. Report your answer with the correct number of digits.

• Undo the scientific notation so you can see what place the “2” actually is in.

1.009 smallest digit is in the thousandths place

+ 1200 smallest digit is in the hundreds place

1201.009 need to keep only hundreds place

= 1200 use scientific notation to avoid ambiguous zeros

= 1.2 x 103 m

Notice: this is not the answer we’d get if we were looking at sig figs instead of decimal places.

Notice: 1.009 is so small compared to the smallest “good” digit in 1200 that it doesn’t impact the answer.

Progress clicker question (covers material we are learning now)Go to LearningCatalytics.com Session ID =

4) Which of the following four problems is not shown with its correct answer?

A) 0.003 + 0.008 = 0.011 C) 3.87 – 1.07 = 2.8

B) 1306 - 4 = 1302 D) 53 + 9 + 78 = 1.40 x 102

9

Background: Calculating formula mass

10

This rule about adding measurements has application when we determine a compound’s formula mass.

Steps:1) Write the chemical formula2) Look up the individual atomic masses from periodic table3) Use the above equation to find the formula mass4) Report the answer with the correct number of decimal places and units (amu)

Textbook error

11

• The “1” and “4” are exact numbers and are short-hand for adding

12.0135.4535.4535.45

+ 35.45153.81

• All of the masses have a 100ths place and therefore so should our answer

Progress clicker question (covers material we are learning now)Go to LearningCatalytics.com Session ID =

5) Calculate the formula mass of lithium perbromate.

A) 150.841 amu D) 151 amu

B) 150.84 amu E) 150 amu

C) 150.8 amu F) 1.5 x 102 amu

12

Answer:

LiBrO4

Notice that this does not match the answer we’d get if we were looking at sig figs instead of decimal places.

formula mass = (1)(6.941) + (1)(79.90) + (4)(16.00)

= (6.941) + (79.90) + (64.00)

= 150.841

Smallest digit is in the thousandths place Smallest digits are in the hundredths place

= 150.84 amu

so, keep 100ths place

Progress clicker question (covers material we are learning now)Go to LearningCatalytics.com Session ID =

6) Calculate the formula mass of antimony(III) hydroxide.

A) 172.824 amu D) 173 amu

B) 172.82 amu E) 170 amu

C) 172.8 amu F) 1.7 x 102 amu

13

Answer:Sb(OH)3

formula mass = (1)(121.8 amu) + (3)(16.00 amu) + (3)(1.008 amu)

= (121.8 amu) + (48.00 amu) + (3.024 amu)

= 172.824 amu (keep the 10ths place)

10ths place 100ths place 1000ths place

= 172.8 amu